High Temperature Performance of Phenolic/ Novolac Epoxies - Thermal Degradation and Barrier Properties

Abstract Maintaining a proper barrier property of the protective coating over time is important to maintain the steel integrity also for assets operating at elevated temperatures. Phenolic/ novolac epoxy coatings are being used as protective barriers for assets operating at elevated temperatures, typically up to 204 °C (400 °F), and there is a large variety of coating products which all fall under the generic description phenolic/ novolac epoxy in the market today. To better understand how these types of coatings perform during static dry heat exposure, two phenolic/ novolac epoxy coating systems were exposed to temperatures of 180 °C (356°F) and 204 °C (400 °F) for a prolonged period. After exposure, all samples were examined visually, mechanically, by optical microscope and by electrochemical impedance spectroscopy (EIS). The results from our investigations indicate different degradation mechanisms over time for the two coating systems even though they are both phenolic/ novolac epoxy coatings. The differences were not clearly visible until after 720 hours of static dry heat exposure, which is the longest duration defined in a widely used standard with clear acceptance criteria utilized by the industry. This study has resulted in deeper knowledge about the phenolic/ novolac epoxy coatings and their high temperature service properties and illustrates the importance of testing at various temperatures for a long duration.